The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in the background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
Mobile telecommunication systems, such as those based on the 3GPP defined UMTS and Long Term Evolution (LTE) architecture, are able to support more sophisticated services than simple voice and messaging services offered by previous generations of mobile telecommunication systems. For example, with the improved radio interface and enhanced data rates provided by LTE systems, a user is able to enjoy high data rate applications such as video streaming and video conferencing on mobile communications devices that would previously only have been available via a fixed line data connection.
The demand to deploy fourth generation networks is therefore strong and the coverage area of these networks, i.e. geographic locations where access to the networks is possible, is expected to increase rapidly.
However, although the coverage and capacity of fourth generation networks is expected to significantly exceed those of previous generations of communications networks, there are still limitations on network capacity and the geographical areas that can be served by such networks. These limitations may, for example, be particularly relevant in situations in which networks are experiencing high load and high-data rate communications between communications devices, or when communications between communications devices are required but the communications devices may not be within the coverage area of a network.
In order to address these limitations, a terminal device (or user equipment, UE) may operate to provide a local cell, the local cell being a smaller cell at least a portion of which is provided within the larger cell of a base station. This UE works simultaneously as an intermediate node between other UEs in its vicinity and the network, as well as an intermediate node between other UEs. It communicates with its UEs in unlicensed, shared licensed or licensed bands, and backhauls to the network (using licensed bands, for example). The local cell is provided at least on a temporary basis by the terminal device.
It is noted that throughout this description, when it is said (for ease of explanation) that a local cell performs an action (such as transmitting, receiving or processing signals, for example), what is actually meant is that it is the UE providing that local cell which performs that action. It is also noted that a local cell may also be referred to as a virtual cell.
It is envisaged that local cells should take responsibilities like radio resource management, radio resource control (RRC) connection control, etc. instead of solely relying on an eNodeB or small cell of the network. Thus, the local cell will not only relay data, but will also help to organize the local network. The existence of such nodes in the network will help to, for example, offload the signalling overhead of the eNodeB (eNB), allocate radio resource efficiently, etc.
Various advantages of establishing an RRC connection between a local cell and UE are listed below.
(1) The RRC signalling overhead between the eNB and UE is reduced. In the conventional network architecture, the eNB should be responsible to maintain an RRC connection with every UE in coverage and the signalling overhead will be non-negligible in the dense UE scenario. One solution to reduce the signalling is to select some local cells located in coverage of the eNB. Each selected local cell will then manage the RRC connections with UEs within its range. With use of the local cell, the eNB will not need to maintain full RRC connections with UEs individually, but will instead only need to keep a partial RRC connection with the UEs.(2) The use of centralized resource allocation within the local cell in order to improve spectrum efficiency and interference mitigation. Compared with single node centralized resource allocation at the eNB, the distributed resource allocation using local cells will have the merits of flexibility and robustness. Through coordination with local cells in the network, the eNB will have better control on the interference mitigation and resource management of the whole network, thus reducing inter-local cell interference. Furthermore, for each local cell, it is relatively easy to manage a smaller group of UEs, with a finer granularity in resources allocated to UEs within the range of the local cell so as to reduce the intra-local cell interference (with finer granularity, the number of resources which may be allocated is increased, and thus the probability of users sharing the same resources is decreased). Thus, each local cell takes responsibility for the resource allocation to each of its UEs. The RRC connection is then managed by the local cell to configure/re-configure the related physical control channels and data channels to receive the resource allocation grant and data, respectively, as well as other configurations to support the resource allocation (for example, buffer status report (BSR) timers, etc.).(3) Mobility control is supported by the local cell in order to guarantee the service continuity. For an RRC_IDLE mode UE, if the UE establishes the RRC connection with the local cell, its position will be tracked by the local cell whether it is in the local cell RRC_idle state (by paging to identify Tracking Area) or in the local cell RRC connected state. Through the local cell (together with the position of the local cell), the eNB could track the UE even when the UE is in RRC_IDLE mode. In addition, in order to keep the service continuity, it is important to support fall back to the network and/or handover to another local cell. The RRC connection of the UE with the local cell supports this requirement.(4) Quality of Service (QoS) differentiation is supported. In future networks, it is important to support QoS differentiation among users as well as among services for each user. For a local cell, managing the resource allocations for different QoS classes is a way of improving the user perceived quality. The local cell should also be responsible for establishing/maintaining/releasing the corresponding radio bearers with the UEs in order to map the logical channel configuration with the services. The RRC connection between the local cell and UE supports the radio bearer establishment/maintenance/release.(5) Measurement report sending is supported in order to improve spectrum efficiency and support service continuity. In order to support local resource allocation by the local cell, the local cell should know the link quality between the local cell and its scheduling UE. The UE needs to measure the channel quality with local cell and report this to the local cell in response to a trigger. The RRC connection between the local cell and UE should be established in order to configure the measurements and send back the measurement report to the local cell.
There are a number of problems with such use of local cells, however.
Firstly, it is noted that a local cell is usually triggered in hot spot area or in an on-demand manner. Both the local cell and UEs may move. For example, consider the following scenarios.
(1) Group split scenario. A local cell is triggered for a metro station in rush hour. Groups of people are walking towards the metro station while the other groups are leaving. For the groups moving towards the local cell, it's better to keep them served by the local cell. On the hand, for the leaving groups, it is better to fall back to the eNB or to handover to another local cell.(2) Group merge scenario. A local cell V1 has been triggered for a metro station in rush hour. The UEs approaching V1 will connect locally with it. Another group of UEs is moving towards V1. A local cell V2 has been triggered to enhance inter-connectivity within the group. As the V2 group comes closer to the range of V1, it is better to merge the V1 and V2 groups.(3) Group moving scenario. On the way home, colleagues in the same office building with a similar home destination may walk together to a metro station, take the same subway line, and walk back home in the same direction. A series of local cell may be triggered along the route and the group of people may handover from one local cell to another.
The commonalties of these groups are summarized as below.
(1) The group members are of geographical vicinity.
(2) The group members move towards approximately the same direction.
(3) The group members have low relative speed to each other.
There is a need to manage these groups of UEs. In particular, the groups need to be managed whilst taking into consideration the above-mentioned group characteristics. Furthermore, this should be achieved with reduced signalling overhead and improved handover efficiency. It is noted that, in particular, a management solution is sought for local cells which are deployed in a fixed way (that is, always there but with no or limited mobility with respect to UEs in its vicinity) and for UEs which have a fixed route or exhibit cluster behaviour. For example, for a local cell in a subway station (which may be, for example, a mobile phone of one or more of the platform staff), the surrounding UEs exhibit cluster behaviour as groups of people get on and off the subway, arrive at the subway station. Also, for a local cell in a moving vehicle (such as a train or bus), the surrounding UEs held by passengers in the vehicle have a fixed route.
There is also a need for a local cell to efficiently use the resources made available to it by the eNB for allocating to UEs connected to the local cell whilst, at the same time, ensuring low communication latency for those UEs. This applies particularly to uplink transmission from the UEs to the local cell.